Plants are subject to a wide variety of diseases caused by assorted microorganisms (fungi, bacteria, and viruses) and damage caused by insects. Fruit bearing plants, in particular citrus trees, are subject to several destructive diseases. The bacterium Xanthomonas axonopodis pv. citri (Xac), the cause of Asiatic citrus canker (Canker), is spread by wind, rain and by mechanical means; and, Huanglongbing/Citrus Greening Disease (Greening), caused by the bacterium Liberibacter asiaticus, is vectored by the Asian citrus psyllid (AsCP), Diaphorina citri Kuwayama. Canker and Greening are particularly problematic for citrus crops as is the insect vector, psyllids. Presently, Greening is prevalent throughout all citrus producing countries in the world with no known cure. Often, governments mandate the removal and destruction of trees identified and affected with Greening.
Hundreds of thousands of acres worldwide have already been destroyed because of Canker. The Brazilian Government currently mandates that citrus trees affected with Greening be removed and burned. North American growers are not required to remove and destroy Greening trees, but are strongly urged to do so by the authorities. However, North American citrus growers could face the same destructive regulatory burdens in the future. In the US, Canker management relies heavily on the prophylactic application of copper-based compounds. These surface-only copper-based sprays have little or no effect because they are easily washed off by moisture requiring frequent reapplications throughout the growing season; thus, this approach to Canker management is inefficient, relatively expensive, and provides minimal protection at best.
Plants possess an arsenal of innate chemical defenses that are activated in response to microbial pathogens or insect pests. A number of phytohormones regulate these innate plant defenses, of which salicylic acid (SA) serves as an internal signaling molecule to systemically activate plant defenses in distal tissues through a process called systemic acquired resistance (SAR). SAR is dependent on the plant's ability to synthesize and accumulate SA that serves to trigger the downstream induction of SA dependent defense genes, including a series of pathogenesis-related (PR) genes. The expression of these PR genes can inhibit pathogen growth within the plants and reduce disease symptoms; this response is also referred to as plant basal defense and also plays an important role in genetic resistance. SAR can be triggered biologically by either subjecting the plant to a limited infection by a weak pathogen or to an avirulent pathogen (one that the plant has a genetic resistance to). However, a number of chemical compounds can also trigger SAR, such as 2,6-dichloroisonicotinic acid (INA) and benzothiadiazole (BTH), and through the exposure of plants to SA itself, or to the salts of SA described herein as the invention. The effectiveness of SAR as triggered chemically by SA, INA, and BTH has been well documented against various plant pathogens in diverse environments (Vallad and Goodman, 2005).
SA (0-hydroxybenzoic acid, CAS: 69-72-7, C6H4(OH)(COOH) is a white powder, of very low solubility, which is commercially available in technical, USP, and crude grade, and not available in any effective, concentrated agricultural solutions for use on commercial crops. SA is only soluble in acetone, oil of turpentine, alcohol, ether, benzene and only slightly soluble in water. Some non fluid salts of SA exist as pharmaceuticals, and chemical test kits, which are not suited for agricultural usage
While SA would be beneficial to plants in effective amounts, the problem has been the difficulty of maintaining SA substantially in solution for plant uptake. Foliar application of unsolubilized SA, and other improperly formulated forms can cause injury to plants.
Presently, there are no formulations of SA, registered either by the Environmental Protection Agency (EPA), or by the Association of American Plant Food Control Officials (AAPFCO), for use on crops in any form.
U.S. Pat. No. 6,649,566 issued to Doostdar discloses a stable aqueous formulation comprising salicylic acid, hydrolyzed chitosan which is a weak base, a strong inorganic base, and humic acid. The inorganic acid is used to stabilize, but not fully solubilize, the salicylic acid; and the humic acid is used to stabilize the chitosan. Doostdar implies a synergistic effect on plants by delivery of salicylic acid/chitosan combination. Although the formulation is disclosed as being a stable solution, it makes to claim to full solubility of the salicylic acid, and the Doostdar reference is silent with respect to extended solubility and shelf life and efficacy of the product. Testing confirms it is not possible to formulate a fully soluble, high analysis aqueous SA solution using only potassium hydroxide as a base for reaction which provides a long shelf life in solution.
It would be desirable to have an effective SA-based product that can be safely applied to fruit bearing plants, without phytoxicity, in a single application that will systemically stop or effectively retard damage caused by fungal and bacterial diseases and insects and provide plant growth hormonal protection to these plants. The present disclosure relates to agricultural plant spray compositions that will stop or effectively retard such damage and provide plant growth hormonal protection in one application.